feat(plugin): load external cdylib plugins at runtime (phase 2)

Move BuiltinPlugin into ocs_plugin_api (host feature) so out-of-tree
crates can implement it, and add export_plugin! to emit the two C
symbols a cdylib exposes: ocs_plugin_api_version (checked first, so an
ABI-incompatible build never runs) and ocs_plugin_register -> boxed
BuiltinPlugin. The host loads every compatible package from the plugins
folder at startup via libloading (desktop only), keeps the library
resident for the session, and merges its ribbon tab + command dispatch
into the same paths as built-ins (honouring enable/disable). The Plugin
Manager shows external packages with a Loaded / incompatible status.

Approach B: the plugin hands back a boxed trait object, assuming a
matching toolchain + ocs_plugin_api version (the version symbol enforces
the latter). crates/ocs_example_plugin is the reference cdylib.

Part of the #100 extensibility epic (phase 2).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/ocs_example_plugin/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "ocs_example_plugin"
+version = "0.1.0"
+edition = "2021"
+description = "Reference external add-on for Open CAD Studio — builds a cdylib the host loads at runtime."
+license = "GPL-3.0-only"
+
+# A dynamically-loaded plugin is a C-ABI dynamic library. `cargo build -p
+# ocs_example_plugin` produces target/<profile>/libocs_example_plugin.so (or
+# .dll/.dylib); drop it beside a plugin.toml in the plugins folder.
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+# The stable contract, with the runtime `host` surface (HostApi/BuiltinPlugin).
+ocs_plugin_api = { path = "../ocs_plugin_api", features = ["host"] }